Well, where is the data coming from? If it's a database, write your query so as to not retrieve duplicate information.
If it's from somewhwere else, why not append the data to a string array first, but before adding to the array, check it against all earlier values saved in the array. If it's already there, don't add a new element. Then, after all are checked, copy the contents of the array to the listbox.
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